Semiconductor device-based sensors and methods associated with the same

ABSTRACT

Semiconductor device-based chemical sensors and methods associated with the same are provided. The sensors include regions that can interact with chemical species being detected. The chemical species may, for example, be a component of a fluid (e.g., gas or liquid). The interaction between the chemical species and a region of the sensor causes a change in a measurable property (e.g., an electrical property) of the device. These changes may be related to the concentration of the chemical species in the medium being characterized.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/879,704, filed Jun. 28, 2004, entitled “Semiconductor Device-BasedSensors and Methods Associated with the Same” which is incorporatedherein by reference.

BACKGROUND OF INVENTION

The invention relates generally to semiconductor device-based sensorsand, more particularly, to gallium nitride material-based transistorsthat function as chemical sensors and methods associated with the same.

Sensors may be used in many applications, for example, to detect theconcentration of a chemical species. The chemical species may be acomponent of a fluid (e.g., a gas or a liquid) to which the sensor isexposed.

Semiconductor devices have been used as chemical sensors. For example,field effect transistor-based sensors may detect the concentration of achemical species by measuring changes in drain current (i.e.,source-to-drain current) that result from adsorption of the chemicalspecies on the surface of the gate electrode. This adsorption may leadto a change in drain current. The change in the drain current may berelated to the concentration of the adsorbed chemical species which, inturn, may be related to the concentration of the chemical species in themedium (e.g., fluid) being characterized.

SUMMARY OF INVENTION

The invention provides semiconductor device-based sensors, as well asmethods associated with the same.

In one embodiment, a FET-based chemical sensor designed to detect achemical species is provided. The sensor comprises a semiconductormaterial region. The sensor further comprises a source electrode, adrain electrode and a gate electrode, each formed on the semiconductormaterial region. The sensor further comprises a sensing region,separated from the gate electrode, and capable of interacting with thechemical species to change a measurable property of the chemical sensor.

In another embodiment, a FET-based chemical sensor designed to detect achemical species is provided. The sensor comprises a silicon substrate;a transition layer formed on the silicon substrate; and, a galliumnitride material region formed on the transition layer. The sensorfurther comprises a source electrode, a drain electrode and a gateelectrode, each formed on the gallium nitride material region. Thesensor further comprises a sensing region, separate from the gateelectrode, and capable of interacting with the chemical species tochange a drain current of the chemical sensor.

In another embodiment, a semiconductor device-based chemical sensor isprovided. The sensor comprises a semiconductor material region; and, asensing electrode formed on the semiconductor material region. Thesensing electrode is separated from electrodes needed for conventionaloperation of the device and is capable of interacting with the chemicalspecies to change a measurable property of the chemical sensor.

In another embodiment, a semiconductor device-based chemical sensor isprovided. The sensor comprises a semiconductor material region. A firstelectrode and a second electrode are formed on the semiconductormaterial region. A first electrical contact extends from a backside ofthe sensor to the first electrode; and a second electrical contactextends from a backside of the sensor to the second electrode. Thesensor further comprises a sensing region, separated from the firstelectrode and the second electrode, and capable of interacting with thechemical species to change a measurable property of the chemical sensor.

In another embodiment, a method of detecting chemical species isprovided. The method comprises exposing a FET-based chemical sensor to amedium comprising chemical species; and measuring changes in draincurrent of the sensor resulting from adsorption of the chemical specieson a sensing region separated from the gate electrode to detect chemicalspecies.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. Theaccompanying figures are schematic and are not intended to be drawn toscale. In the figures, identical, or substantially similar componentsthat are illustrated in various figures may be represented by a singlenumeral or notation. For purposes of clarity, not every component islabeled in every figure. Nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow thoseof ordinary skill in the art to understand the invention. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict withincorporated references, the present specification, includingdefinitions, will control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-section of a field effect transistor (FET)-basedchemical sensor including a sensing electrode according to an embodimentof the invention.

FIG. 1B is a top view of the sensor of FIG. 1A taken along line 1B-1B inFIG. 1A.

FIG. 2A is a cross-section of a FET-based chemical sensor including anexposed sensing region according to an embodiment of the invention.

FIG. 2B is a top view of the sensor of FIG. 2A taken along line 2B-2B inFIG. 2A.

FIG. 3 is a cross-section of a FET-based chemical sensor including twosensing electrodes according to an embodiment of the invention.

FIG. 4 is a cross-section of a FET-based chemical sensor including twosensing electrodes respectively located on opposite sides of the sensoraccording to an embodiment of the invention.

FIG. 5 is a cross-section of a FET-based chemical sensor including twosensing electrodes respectively located on opposite sides of the sensoraccording to an embodiment of the invention.

FIG. 6 is a cross-section of a FET-based chemical sensor having backsideelectrical contacts according to an embodiment of the invention.

FIGS. 7A and 7B respectively are a top view and a cross-section of aSchottky diode-based chemical sensor according to an embodiment of theinvention.

DETAILED DESCRIPTION

The invention provides semiconductor device-based chemical sensors andmethods associated with the same. The sensors include a sensing regionthat can interact with chemical species being detected. The chemicalspecies may, for example, be a component of a fluid (e.g., gas orliquid). The interaction between the chemical species and a region ofthe sensor causes a change in a measurable property (e.g., an electricalproperty such as drain current) of the device. These changes may berelated to the concentration of the chemical species in the medium beingcharacterized. As described further below, the sensing region may be thecombination of a sensing layer (e.g., a sensing electrode) and anunderlying semiconductor material region, or an exposed semiconductormaterial (e.g., gallium nitride material) region. Sensors of theinvention may be used in a wide variety of applications including engineemission monitoring, carbon dioxide detection and flue gas monitoring,amongst others.

FIGS. 1A and 1B illustrate a semiconductor device-based sensor 10according to one embodiment of the invention. In the illustrativeembodiment, the sensor is a field effect transistor (FET) that includesa source electrode 12, a drain electrode 14 and a gate electrode 16formed on a semiconductor material (e.g., gallium nitride material)region 18. The semiconductor material region is formed on a substrate 20and, as shown, a transition layer 22 may be formed between the substrateand the semiconductor material region. The sensor includes a sensingregion 23 between the source and the drain electrodes. In theillustrative embodiment, the sensing region comprises a sensing layer 24and an underlying surface region 25 of the semiconductor materialregion. As shown, the sensing layer is a sensing electrode which, alongwith the gate electrode is defined, at least in part, by anelectrode-defining layer 26.

During use, sensing region 23 is exposed to a medium (e.g., a fluid)that includes chemical species to be detected. As described furtherbelow, the chemical species in the medium (e.g., by following thedirection of the schematic arrows in FIG. 1A) are adsorbed by thesensing layer which leads to an interaction between the species and thesensing layer. It is believed that this interaction results in a changein the surface potential of underlying surface region 25. This change insurface potential results in a change in a property (e.g., draincurrent) of the device which may be measured and related to theconcentration of the species in the medium.

In some cases, it is believed that the chemical species may also diffusethrough the sensing layer and directly interact with the underlyingsemiconductor material region 18. However, it should be understood thatthe invention is not limited to a particular mechanism by which themeasurable property (e.g., drain current) of the device is changedand/or by which the surface potential of the underlying surface regionchanges. In embodiments of the invention, the chemical species interactswith a sensing region in some manner which results in the propertychange.

Though sensor 10 is an FET in the illustrative embodiment of FIGS. 1Aand 1B, it should be understood that the invention encompasses othertypes of semiconductor device-based sensors as described further below.

When a layer is referred to as being “on” or “over” another layer orsubstrate, it can be directly on the layer or substrate, or anintervening layer may also be present. A layer that is “directly on”another layer or substrate means that no intervening layer is present.It should also be understood that when a layer is referred to as “on” or“over” another layer or substrate, it may cover the entire layer orsubstrate, or a portion of the layer or substrate.

Sensing layer 24 may be formed of any suitable material that is capableof interacting with a chemical species being detected in a manner thatresults in a change in a measurable property (e.g., drain current) ofthe sensor. In some cases, the sensing layer is formed of a conductivematerial (e.g., when the sensing layer is a sensing electrode). Suitableconductive sensing layer materials include, but are not limited to,metals, metal compounds and doped semiconductor materials. In someembodiments, the sensing layer comprises a noble metal (or one of itsalloys) or a transition metal (or one of its alloys). For example, thesensing layer may comprise one or more of the following metals:platinum, palladium, iridium, ruthenium, nickel, copper, rhodium,molybdenum, iron, cobalt, titanium, vanadium, tantalum, tungsten,chromium, magnesium, gold, silver, aluminum, tin, osmium, magnesium,zinc, as well as alloys of these metals and compounds of these metals.

It should also be understood that the above-described conductivematerials may be suitable when the sensing layer is not functioning asan electrode.

In other cases (e.g., when the sensing layer is not functioning as anelectrode), the sensing layer may be formed of non-conductive materialssuch as dielectric materials or polymeric materials.

In some embodiments, the sensing layer may comprise more than one layer,each having a different composition. In some embodiments, one or morelayer may be formed above the sensing layer, or between the sensinglayer and the semiconductor material region.

The sensing layer material may be selected for its sensitivity to aparticular chemical species. In cases when a highly sensitive sensor fora specific chemical species is desired, the sensing layer may preferablycomprise a material that interacts with the chemical species in a mannerthat causes a relatively large change in surface potential of theunderlying surface region. Suitable chemical species/materialcombinations are known in the art. For example, platinum or palladiumelectrodes are particularly well suited in hydrogen sensors; and, tinoxide is particularly well suited in CO₂ or CO detectors.

As shown in FIGS. 1A and 1B, the sensing layer may be a sensingelectrode that is connected to a voltage source. When the sensing layer24 is an electrode, it should be formed of a material that issufficiently conductive. However, it should be understood that, in otherembodiments, the sensing layer is not an electrode.

The sensing layer may have any suitable dimensions that enable thedevice to function as desired. Because the sensing layer is separatedfrom the gate electrode, the dimensions and design of the sensing layer(e.g., when functioning as a sensing electrode) are not subject to anygate electrode design requirements. This increases the ability to tailorthe dimensions of the sensing layer (e.g., when functioning as a sensingelectrode) to improve sensor performance. In certain conventionalsensors in which the gate electrode also functions as a sensingelectrode (i.e., sensors that measure changes in drain current thatresult from adsorption of the chemical species on the surface of thegate electrode) there may be less freedom over the dimensions of thiselectrode because it must satisfy gate electrode requirements. Thus,because of the added dimensional freedom, sensors of the invention mayhave sensing layers that can be tailored to have improved sensorperformance (i.e., sensitivity, responsivity, and recovery) compared tosuch conventional sensors.

For example, sensing layer 24 may be thinner than typically possible inconventional sensors in which the gate electrode also functions as asensing electrode. Thin sensing electrodes permit relatively rapiddiffusion of chemical species through the electrode to surface region 25which can improve sensitivity and responsivity of the sensor. Also, thinsensing layers enable rapid out diffusion of chemical species which maylead to improved sensor recovery times. For example, the sensing layermay have a thickness of less than about 500 nanometers such as betweenabout 5 nanometers and about 200 nanometers, or between about 50nanometers and about 200 nanometers. In other embodiments, the sensinglayers may be thicker and have a thickness of greater than about 500nanometers.

The surface area of the sensing layer is designed to enable sufficientamounts of the chemical species to be adsorbed during operation. It maybe advantageous in highly sensitive sensor applications for sensinglayers to have a relatively large surface area when compared to thetotal channel surface area (i.e., the area defined between the sourceelectrode and the drain electrode) of the device. In some cases, theratio of the sensing electrode surface area to the total channel surfacearea is greater than about 0.35. In some cases, the ratio of the sensingelectrode surface area to the total channel surface area is betweenabout 0.20 and about 0.95; and, in some cases, between about 0.35 andabout 0.60.

As noted above, in the embodiments of FIGS. 1A and 1B, sensing layer 24is separated from gate electrode 16 so that the sensor includes fourdifferent electrodes when the sensing layer is a sensing electrode.Thus, an external voltage may be applied independently to each of theelectrodes (i.e., source, gate, drain and sensing). Though, it shouldalso be understood that the same external voltage may be applied to morethan one of the different electrodes.

The ability to independently control the voltages applied to differentelectrodes (and, in particular, independently controlling the voltageapplied to the sensing electrode and the gate electrode) may enhance thesensitivity and/or concentration range of the sensor for detecting achemical species. In particular, the sensitivity and/or range may beenhanced as compared to conventional sensors that include a gateelectrode that also functions as a sensing electrode because of theadditional degree of freedom provided when the sensing electrode isseparate from the gate electrode. In the embodiment of FIGS. 1A and 1B,the voltages applied to the different electrodes may be selected suchthat the dependency of the drain current on the concentration ofadsorbed species is increased or maximized. In one preferred mode ofoperation, the gate electrode is biased near the maximumtransconductance of the FET device while the source is grounded and thedrain is positively biased. In this mode, small changes in the surfacepotential of the surface region 25 may translate into relatively largechanges in drain current which results in high sensitivity.

In the illustrative embodiment, sensing region 23 is separated from thegate electrode which means that the sensing region is located at adifferent position on the device than the gate electrode and the regiondirectly beneath the gate electrode. Thus, the sensing region islaterally separated from the gate electrode in the illustrativeembodiment. As shown, the sensing region is positioned between thesource and the drain electrodes and, more particularly, between the gateand drain electrodes. It should be understood that the sensing regionmay be positioned in other locations including between the source andgate electrodes.

In certain preferred embodiments, substrate 20 is a silicon substrate.As used herein, a silicon substrate refers to any substrate thatincludes a silicon surface. Examples of suitable silicon substratesinclude substrates that are composed entirely of silicon (e.g., bulksilicon wafers), silicon-on-insulator (SOI) substrates,silicon-on-sapphire substrate (SOS), and SIMOX substrates, amongstothers. Suitable silicon substrates also include substrates that have asilicon wafer bonded to another material such as diamond, AlN, or otherpolycrystalline materials. Silicon substrates having differentcrystallographic orientations may be used. In some cases, silicon (111)substrates are preferred. In other cases, silicon (100) substrates arepreferred.

It should be understood that other types of substrates may also be usedincluding sapphire, silicon carbide, indium phosphide, silicongermanium, gallium arsenide, gallium nitride material, aluminum nitride,or other III-V compound substrates. However, in embodiments that do notuse silicon substrates, all of the advantages associated with siliconsubstrates may not be achieved.

It should also be understood that though the illustrative embodimentsinclude a substrate, other embodiments of the invention may not have asubstrate. In these embodiments, the substrate may be removed duringprocessing. In other embodiments, the substrate may also function as thesemiconductor material region. That is, the substrate and semiconductormaterial region are the same region. For example, in these embodiments,the substrate (and semiconductor material region) may be formed of agallium nitride material.

Substrate 20 may have any suitable dimensions. Suitable wafer diametersinclude, but are not limited to, 2 inches (50 mm), 4 inches (100 mm), 6inches (150 mm), and 8 inches (200 mm). The diameter of the waferdepends, in part, on the type of substrate and the composition of thesemiconductor material region. In some cases, it may be preferable forthe substrate to be relatively thick, such as greater than about 125micron (e.g., between about 125 micron and about 800 micron, or betweenabout 400 micron and 800 micron). Relatively thick substrates may beeasy to obtain, process, and can resist bending which can occur, in somecases, in thinner substrates. In other embodiments, thinner substrates(e.g., less than 125 microns) are used, though these embodiments may nothave the advantages associated with thicker substrates, but can haveother advantages including facilitating processing and/or reducing thenumber of processing steps. In some processes, the substrate initiallyis relatively thick (e.g., between about 200 microns and 800 microns)and then is thinned during a later processing step (e.g., to less than150 microns).

In some preferred embodiments, the substrate is substantially planar inthe final device or structure. Substantially planar substrates may bedistinguished from substrates that are textured and/or have trenchesformed therein (e.g., as in U.S. Pat. No. 6,265,289). In theillustrative embodiments, the regions/layers formed on the substrate(e.g., transition layer, gallium nitride material region, and the like)are also substantially planar. As described further below, suchregions/layers may be grown in vertical (e.g., non-lateral) growthprocesses. Planar substrates and regions/layers can be advantageous insome embodiments, for example, to simplify processing. Though it shouldbe understood that, in some embodiments of the invention, lateral growthprocesses may be used as described further below.

Transition layer 22 may optionally be formed on substrate 20 prior tothe deposition of semiconductor material (e.g., gallium nitridematerial) region 18. The presence of the transition layer depends, inpart, on the type of substrate and the composition of the semiconductormaterial region. When the substrate is a silicon substrate and thesemiconductor material region is a gallium nitride material region, thepresence of the transition layer may be preferred.

The transition layer may accomplish one or more of the following:reducing crack formation in the semiconductor material (e.g., galliumnitride material) region by lowering thermal stresses arising fromdifferences between the thermal expansion rates of the semiconductormaterial region and the substrate; reducing defect formation insemiconductor material region by lowering lattice stresses arising fromdifferences between the lattice constants of the semiconductor materialregion and the substrate; and, increasing conduction between thesubstrate and semiconductor material region by reducing differencesbetween the band gaps of substrate and gallium nitride materials. Thepresence of the transition layer may be particularly preferred whenutilizing silicon substrates and gallium nitride material regionsbecause of the large differences in thermal expansion rates and latticeconstants between gallium nitride materials and silicon. It should beunderstood that the transition layer also may be formed betweensubstrate 20 and gallium nitride material region for a variety of otherreasons. In some cases, for example when a silicon substrate is notused, the device may not include a transition layer.

The composition of transition layer 22 depends, at least in part, on thecompositions of the substrate and of the semiconductor material region.In some embodiments which include a silicon substrate and a galliumnitride material region, the transition layer may preferably comprise acompositionally-graded transition layer having a composition that isvaried across at least a portion of the layer. Suitablecompositionally-graded transition layers, for example, have beendescribed in commonly-owned U.S. Pat. No. 6,649,287, entitled “GalliumNitride Materials and Methods,” filed on Dec. 14, 2000, which isincorporated herein by reference. Compositionally-graded transitionlayers are particularly effective in reducing crack formation in thegallium nitride material region by lowering thermal stresses that resultfrom differences in thermal expansion rates between the gallium nitridematerial and the substrate (e.g., silicon).

According to one set of embodiments, the transition layer iscompositionally graded and formed of an alloy of gallium nitride such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. Inthese embodiments, the concentration of at least one of the elements(e.g., Ga, Al, In) of the alloy is varied across at least a portion ofthe thickness of the transition layer. When the transition layer has anAl_(x)In_(y)Ga_((1-x-y))N composition, x and/or y may be varied. Whenthe transition layer has a Al_(x)Ga_((1-x))N composition, x may bevaried. When the transition layer has a In_(y)Ga_((1-y))N composition, ymay be varied.

In certain preferred embodiments, it is desirable for the transitionlayer to have a low gallium concentration at a back surface which isgraded to a high gallium concentration at a front surface. It has beenfound that such transition layers are particularly effective inrelieving internal stresses within the gallium nitride material region.For example, the transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is decreased from the back surface to thefront surface of the transition layer (e.g., x is decreased from a valueof 1 at the back surface of the transition layer to a value of 0 at thefront surface of the transition layer). The composition of thetransition layer, for example, may be graded discontinuously (e.g.,step-wise) or continuously. One discontinuous grade may include steps ofAlN, Al_(0.6)Ga_(0.4)N and Al_(0.3)Ga_(0.7)N proceeding in a directiontoward the semiconductor material (e.g., gallium nitride material)region.

In some cases, the transition layer has a monocrystalline (i.e., singlecrystal) structure. In some embodiments, transition layer 22 has aconstant (i.e., non-varying) composition across its thickness.

In some embodiments, sensors of the invention may also optionallyinclude other layers that are not depicted in the figures. For example,the sensor may include one or more intermediate layers (e.g., astrain-absorbing layer). One or more intermediate layers may be formed,for example, between the substrate and the transition layer (e.g., acompositionally-graded transition layer) and/or between the transitionlayer and the semiconductor material region. These layers may have aconstant composition.

Suitable intermediate layers, for example, have been described in U.S.Pat. No. 6,649,287, which is incorporated by reference above. In someembodiments, an intermediate layer may be formed of a nitride-basedcompound such as gallium nitride alloy (such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, or In_(y)Ga_((1-y))N),aluminum nitride, or aluminum nitride alloys.

Suitable strain-absorbing layers (e.g., silicon nitride material-basedlayers), for example, have been described in commonly-owned, co-pendingU.S. patent application Ser. No., not yet assigned, entitled “GalliumNitride Materials and Methods Associated With the Same”, filed Jun. 28,2004, which is incorporated herein by reference.

In some cases, the intermediate layer(s) have a monocrystallinestructure. In other cases, the intermediate layer(s) may have anamorphous structure (e.g., when the intermediate layer is astrain-absorbing layer comprising a silicon nitride-based material).

As noted above, in some embodiments of the invention, it is preferablefor semiconductor material region 18 to comprise a gallium nitridematerial, at least in the area of surface region 25. Gallium nitridematerials have electrical properties (e.g., high piezoresistivity) thatmake them well suited for use in certain sensors of the presentinvention. For example, the surface potential of a gallium nitridematerial region may be strongly dependent on the adsorption of chemicalspecies by the sensing region. This strong dependency can enable thesensor to be highly sensitive.

Gallium nitride materials are also generally able to withstand extremeconditions (such as high temperatures and/or corrosive environments)which certain other semiconductor materials cannot withstand. Thus,sensors that include gallium nitride material semiconductor regions maybe able to operate under certain extreme conditions (and, thus, incertain applications) that sensors that include semiconductor regionsformed of materials other than gallium nitride material.

As used herein, the phrase “gallium nitride material” refers to galliumnitride (GaN) and any of its alloys, such as aluminum gallium nitride(Al_(x)Ga_((1- x))N), indium gallium nitride (In_(y)Ga_((1-y))N),aluminum indium gallium nitride (Al_(x)In_(y)Ga_((1-x-y))N), galliumarsenide phosporide nitride (GaAs_(a)P_(b) N_((1-a-b))), aluminum indiumgallium arsenide phosporide nitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), amongst others. Typically, when present, arsenic and/orphosphorous are at low concentrations (i.e., less than 5 weightpercent). In certain preferred embodiments, the gallium nitride materialhas a high concentrations of gallium and nitrogen. In high galliumconcentration embodiments, the sum of (x+y) may be less than 0.4, lessthan 0.2, less than 0.1, or even less. In some cases, it is preferablefor the gallium nitride material layer to have a composition of GaN(i.e., x+y=0) or Al_(x)Ga_((1-x))N (y=0). In some cases, when thegallium nitride material layer has a composition of Al_(x)Ga_((1-x))N, xis less than about 0.4. Gallium nitride materials may be doped n-type orp-type, or may be intrinsic. Suitable gallium nitride materials havebeen described in U.S. Pat. No. 6,649,287, incorporated by referenceabove.

As shown in certain embodiments, semiconductor material region mayinclude multiple semiconductor layers (e.g., 18 a, 18 b, 18 c) Layer 18b may have a different composition than layer 18 a. For example, layer18 b may be formed of a first gallium nitride material and layer 18 amay be formed of a second gallium nitride material. In certainembodiments, it may be preferable for the gallium nitride material oflayer 18 b to have an aluminum concentration that is greater than thealuminum concentration of the gallium nitride material of layer 18 a.For example, the value of x in the gallium nitride material of layer 18b (with reference to any of the gallium nitride materials describedabove) may have a value that is between 0.05 and 1.0 greater than thevalue of x in the gallium nitride material of layer 18 a, or between0.05 and 0.5 greater than the value of x in the gallium nitride materialof layer 18 a. For example, layer 18 b may be formed ofAl_(0.26)Ga_(0.74)N, while layer 18 a is formed of GaN. This differencein aluminum concentration may lead to formation of a highly conductiveregion at the interface of the layers 18 b, 18 a (i.e., a 2-D electrongas region) which can increase the sensitivity of the surface potentialof surface region 25 which, in turn, can increase the sensitivity of thesensor. In the illustrative embodiment, layer 18 c may be formed of GaN.

In some embodiments, semiconductor material region 18 may be formed ofonly a single semiconductor material. Or, region 18 may be formed of asemiconductor material layer and other non-semiconductor layersincluding oxide layers and/or metallic layers. However, in theseembodiments, it may be preferable for surface region 25 to comprisegallium nitride material to achieve the advantages associated withgallium nitride material described above.

It should also be understood that, in certain embodiments of theinvention, semiconductor material region 18 may be formed primarily, orentirely, of semiconductor materials other than gallium nitridematerial. For example, the semiconductor material region may be formedof silicon, gallium arsenide, or other III-V compounds. However, atleast some of the advantages associated with gallium nitride materialsmay not be achieved when using other semiconductor materials.

Semiconductor material region 18 is of high enough quality to permitoperation of the sensor device. Preferably, the semiconductor material(e.g., gallium nitride material) region has a low crack level and a lowdefect level. As described above, transition layer 22 (particularly whencompositionally-graded) may reduce crack and/or defect formation. Insome embodiments, the semiconductor material region has a defect densityof less than about 10 ⁹ defects/cm².

Gallium nitride materials having low crack levels have been described inU.S. Pat. No. 6,649,287 incorporated by reference above. In some cases,the semiconductor region may comprise gallium nitride materials having acrack level of less than 0.005 micron/micron² or a crack level of lessthan 0.001 micron/micron². In certain cases, it may be preferable forthe semiconductor region to comprise gallium nitride material that issubstantially crack-free as defined by a crack level of less than 0.0001micron/micron².

In certain cases, the semiconductor material region includes a layer orlayers which have a monocrystalline (i.e., single crystal) structure. Incases when the semiconductor region comprises gallium nitride material,the gallium nitride material may have a single crystal structure such asa Wurtzite (hexagonal) structure.

The thickness of semiconductor material region 18 and the number ofdifferent layers are dictated, at least in part, by the requirements ofthe specific device. At a minimum, the thickness of the semiconductormaterial region is sufficient to permit formation of the desiredstructure or device (e.g., FET). The semiconductor region may have athickness of greater than 0.1 micron, though not always. In other cases,the semiconductor material region has a thickness of greater than 0.5micron, greater than 2.0 microns, or even greater than 5.0 microns.

As noted above, in the illustrative embodiment, electrode-defining layer26 defines, at least in part, gate electrode 16 and sensing layer 24(which is a sensing electrode). A first via 30 a is formed in theelectrode-defining layer in which the gate electrode is, in part,formed. The shape and the dimensions of the first via and, thus the gateelectrode, can be controlled to improve certain electrical properties ofthe device. A second via 30 b is formed in the electrode-defining layerin which the sensing layer is, in part, formed. The shape and thedimensions of the second via and, thus the sensing layer, can becontrolled as desired.

Suitable compositions for the electrode-defining layer include, but arenot limited to, nitride-based compounds (e.g., silicon nitridecompounds), oxide-based compounds (e.g., silicon oxide compounds),polyimides, other dielectric materials, or combinations of thesecompositions (e.g., silicon oxide and silicon nitride). In some cases,it may be preferable for the electrode-defining layer to be a siliconnitride compound (e.g., Si₃N₄) or non-stoichiometric silicon nitridecompounds.

In the illustrative embodiment, the electrode-defining layer is directlyon semiconductor material region 18 and functions as a passivating layerthat protects and passivates the surface of the semiconductor materialregion (e.g., gallium nitride material) region.

Suitable electrode-defining layers and gate electrode structures havebeen described in commonly owned, co-pending U.S. patent applicationSer. No. 10/740,376, filed on Dec. 17, 2003, and entitled “GalliumNitride Material Devices Including an Electrode-Defining Layer andMethods of Forming the Same”, which is incorporated herein by reference.

It should be understood that, in certain embodiments, the sensors of theinvention may not include a layer that defines the gate electrode and/orthe sensing layer. In these cases, the gate electrode and/or sensinglayer are otherwise patterned to form the desired structure.

The gate, source and drain electrodes may be formed of any suitableconducting material such as metals (e.g., gold, nickel), metal compounds(e.g., WSi, WSiN), alloys, semiconductors, or combinations of thesematerials. For example, the gate electrode may be formed of gold, nickelor both. Source and drain electrodes may be formed of gold, nickel,titanium, aluminum, platinum or silicon. Such compositions are known tothose of ordinary skill in the art.

In some embodiments, the electrodes may extend into the semiconductormaterial region. For example, electrode material (e.g., metal) depositedon the surface of the gallium nitride material region may diffuse intothe semiconductor material region during a subsequent annealing step(e.g., RTA) when forming the electrode. In particular, the source anddrain electrodes may include such a portion diffused into thesemiconductor material region. As used herein, such electrodes are stillconsidered to be formed on the semiconductor material region.

The sensor shown in FIGS. 1A and 1B includes an encapsulation layer 32which, as known to those of skill in the art, encapsulates underlyinglayers of the structure to provide chemical and/or electricalprotection. The encapsulation layer may be formed of any suitablematerial including oxides or nitrides.

An opening 34 is formed in the encapsulation layer to allow chemicalspecies access to the sensing electrode. In some embodiments, theopening may has sloped sidewalls so that the cross-sectional area of theopening decreases in a direction toward the sensing electrode which mayenhance access to the sensing electrode.

In the illustrative embodiment, interconnects 35 are provided asconductive pathways that are connected to electrodes of the sensor.Though the illustrated interconnects are connected to the source anddrain electrodes, it should also be understood that other conductiveinterconnects (not shown) may be connected to the other electrodes(e.g., gate and sensing). Bond wires 37 are generally provided toelectrically connect the interconnects to a voltage source, thus,providing a conductive pathway from the voltage source to the electrodeson the device.

In the illustrative embodiment, amorphized regions 36 electricallyisolate devices from adjacent devices (not shown). The amorphizedregions may be formed by implanting a species (e.g., nitrogen ions) asdescribed in commonly-owned, co-pending U.S. patent application Ser. No.not yet assigned, entitled “Gallium Nitride Material StructuresIncluding Isolation Regions and Methods”, filed Jun. 28, 2004. It shouldbe understood that other techniques for isolating adjacent devices maybe utilized. In some cases, it may not be necessary to isolate adjacentdevices and, thus, amorphized regions (or other features that isolateadjacent devices) may not be present.

FIGS. 2A and 2B illustrate a semiconductor device-based sensor 40according to another embodiment of the invention. Sensor 40 includes asensing region 42 which is formed of an exposed surface of semiconductormaterial (e.g., gallium nitride material) region 18. During operation,in this embodiment, the chemical species are adsorbed directly onsensing region 42. It is believed that, in these embodiments, thesurface potential of the semiconductor material (e.g., gallium nitridematerial) is changed by the adsorption of the chemical species. Thischange in surface potential leads to a change in a measurable property(e.g., drain current) of the device. As described above, by measuringchanges in this property, it is possible to determine the concentrationof the species adsorbed and, accordingly, the concentration of thespecies in the medium in which the sensor is placed.

One advantage associated with the embodiments of FIGS. 2A and 2B is thatdirect modification of the surface energy of the semiconductor materialsurface, particularly when the semiconductor material is a galliumnitride material, can lead to increased sensitivity, responsivity andrecovery of the sensor. These improved properties arise because thesurface potential of the semiconductor material (particularly, when thesemiconductor material is a gallium nitride material) can be stronglydependent on the concentration of adsorbed chemical species and also thedrain current can be strongly dependent on the surface potential of thesemiconductor material region. Thus, adsorption of even a smallconcentration of chemical species can change the surface potential ofthe semiconductor region which causes measurable changes in draincurrent. In some of these embodiments, these sensor properties may beimproved relative to certain sensors that do not involve directadsorption of the chemical species by a semiconductor material regionbut may involve adsorption by another feature of the structure (e.g., anelectrode).

In the embodiments of FIGS. 2A and 2B, the composition of semiconductormaterial region 18 is selected so that its surface potential isappropriately sensitive to the adsorbed chemical species. Suitablecompositions have been described above in connection with FIGS. 1A and1B including suitable compositions of layers 18 b and 18 a.

Suitable surface areas for region 42 and ratios of surface area to totaldevice surface area have been described above in connection with FIGS.1A and 1B.

FIG. 3 illustrates a semiconductor device-based sensor 50 according toanother embodiment of the invention. In this illustrative embodiment,sensor 50 includes a first sensing layer 24 a and a second sensing layer24 b, both formed between the source electrode and the drain electrode.One, both, or neither of sensing layers 24 a, 24 b may be sensingelectrodes. The first and second sensing layers may be used to detectthe same or different chemical species. In some embodiments, the firstand the second sensing layers need not be placed on the same side of thestructure, as described further below.

FIG. 4 illustrates a semiconductor device-based sensor 60 according toanother embodiment of the invention. Sensor 60 includes a first sensinglayer 24 c formed on a frontside of the sensor and second sensing layer24 d formed on a backside of the sensor. In this illustrativeembodiment, the second sensing layer may be associated with a seconddevice (not shown) (e.g., a second FET) formed in, or in regions belowthe substrate which is separate from the illustrated device formed overthe substrate. Providing a sensor that includes two devices mayfacilitate detection of two species by having the first device detect afirst species and the second device detect the second species.

FIG. 5 illustrates a semiconductor device-based sensor 62 according toanother embodiment of the invention. Sensor 62 includes a first sensinglayer 24 e formed on a frontside of the sensor and second sensing layer24 f formed in a via 64 that extends from a backside of the sensor. Inthis illustrative embodiment, the second sensing layer may be associatedwith the FET device formed over the substrate. That is, the secondsensing layer may interact with chemical species which leads to a changein the surface potential of semiconductor material region 18, asdescribed above. Via 64 may also enhance heat removal from the sensorwhich may be important during operation of the device. Suitable backsidevias have been described in commonly-owned U.S. Pat. No. 6,611,002, andin commonly-owned, co-pending U.S. patent application Ser. No.09/792,409, entitled “Gallium Nitride Materials Including ThermallyConductive Regions”, filed Feb. 23, 2001, by Borges et. al., both ofwhich are incorporated herein by reference.

FIG. 6 illustrates a semiconductor device-based sensor 65 according toanother embodiment of the invention. In this embodiment, vias 66 a, 66 b(and others not shown) extend from a backside of the sensor.Electrically conductive contacts 67 a, 67 b (and others not shown) aredeposited in respective vias 66 a, 66 b which are respectively connectedto the source electrode 12 and drain electrode 14. Contacts 67 a, 67 bextend onto the backside of the device. It should be understood thatother vias and electrically conductive contacts (both not shown) may besimilarly formed for connection to gate electrode 16 and sensing layer24 (when functioning as a sensing electrode). Bond wires 37 are alsoprovided for connecting the backside contacts to respective voltagesources.

Suitable backside vias have been described in commonly-owned U.S. Pat.No. 6,611,002 and U.S. patent application Ser. No. 09/792,409, both ofwhich are incorporated herein by reference above.

By providing electrically connection to the backside of the device, itis possible to eliminate contacts and bond wires from the frontside ofthe device. Thus, in these embodiments, the frontside of the device maybe free of contacts and bond wires. This enables all areas on thefrontside of the device, except for the sensing region, to beencapsulated by encapsulation layer 32. Thus, the encapsulation layermay provide chemical protection for the entire frontside of the device(except for the sensing region). This can be advantageous inapplications in which the topside of the device is exposed to acorrosive environment that would otherwise damage electrical contacts,bond wires or others non-encapsulated regions (e.g., proximateelectrical contacts) when located on the frontside of the device.Positioning electrical contacts at the backside may also facilitatedevice packaging.

FIGS. 7A and 7B illustrate a semiconductor device-based sensor 70according to another embodiment of the invention. In this embodiment,sensor 70 is a Schottky diode that include a Schottky electrode 72 andan ohmic electrode 74 that is formed around the diameter of thestructure. The sensor further includes a sensing layer 24 g (which maybe an electrode) between the Schottky electrode and the ohmic electrodewhich adsorbs chemical species as described above in connection with theother embodiments. As described above, the adsorption of the chemicalspecies results in a change of a measurable property of the sensor(e.g., current).

It should also be understood that, in the embodiments of FIGS. 3-7, thesensing layer(s) may be replaced with a sensing region similar to thoseillustrated and described above in connection with FIGS. 2A and 2B.

It should be understood that sensors of the invention may be based onother types of semiconductor devices than those illustrated in FIGS.1-5. For example, other semiconductor devices that have measurableproperties that may change in response to adsorption of a chemicalspecies may be utilized.

In general, sensors of the invention may be based on devices thatinclude a sensing electrode, separate from electrodes needed forconventional operation of the device. As described above in connectionwith the FET embodiment of FIGS. 1A and 1B, the sensing layer may be asensing electrode separate from the source, drain and gate electrodes.As described above in connection with the Schottky diode embodiment ofFIGS. 7A and 7B, the sensing layer may be a sensing electrode separatefrom the Schottky electrode and the ohmic electrode.

It should also be understood that the chemical sensor may be integratedwith other types of devices (e.g., a pressure sensor) in certainembodiments of the invention.

Sensors of the present invention may be used in a wide variety ofapplications including engine emission monitoring, carbon dioxidedetection and flue gas monitoring, amongst others. Also, the sensors ofthe present invention may be used to detect a wide variety of chemicalspecies including, but not limited to, carbon dioxide, carbon monoxide,hydrogen, oxygen, nitrogen, nitrous oxide, hydrocarbons, alcohols andionic species, amongst others. In some embodiments of the invention, itmay be preferred for the sensors to detect the concentration of gaseousspecies.

As noted above, during use, the chemical species being detected interactwith a sensing region to create a change in the surface potential of anunderlying semiconductor material region with this change in surfacepotential resulting in a change in a property (e.g., drain current) ofthe device. By measuring the change in the property over time, it ispossible to determine the concentration of the species in the medium inwhich the sensor is placed.

Sensors of the present invention may be formed using methods that employconventional processing techniques. For example, the layers of thestructure may be deposited, patterned and etched using conventionaltechniques.

Transition layer 22 and semiconductor material (e.g., gallium nitridematerial) region 18 may be deposited, for example, using metal organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), andhydride vapor phase epitaxy (HVPE), amongst other techniques. Thepreferred technique may depend, in part, on the composition of thelayers. In some cases in which the semiconductor material regioncomprises gallium nitride material, an MOCVD process may be preferred. Asuitable MOCVD process to form a transition layer (e.g., acompositionally-graded transition layer) and gallium nitride materialregion over a silicon substrate has been described in U.S. Pat. No.6,649,287 incorporated by reference above. When the semiconductormaterial region has different layers, in some cases it is preferable touse a single deposition step (e.g., an MOCVD step) to form the entireregion 18. When using the single deposition step, the processingparameters are suitably changed at the appropriate time to form thedifferent layers. In certain preferred cases, a single growth step maybe used to form the transition layer and the semiconductor material(e.g., gallium nitride material) region.

In other embodiments of the invention (not shown), it is possible togrow a semiconductor material (e.g., gallium nitride material) regionusing a lateral epitaxial overgrowth (LEO) technique that involvesgrowing an underlying semiconductor material layer through mask openingsand then laterally over the mask to form the region, for example, asdescribed in U.S. Pat. No. 6,051,849, which is incorporated herein byreference.

In other embodiments of the invention (not shown), it is possible togrow the semiconductor material (e.g., gallium nitride material) regionusing a pendeoepitaxial technique that involves growing sidewalls ofposts into trenches until growth from adjacent sidewalls coalesces toform a semiconductor material region, for example, as described in U.S.Pat. No. 6,265,289, which is incorporated herein by reference. In theselateral growth techniques, semiconductor material (e.g., gallium nitridematerial) regions with very low defect densities are achievable. Forexample, at least a portion of the semiconductor material (e.g., galliumnitride material) region may have a defect density of less than about10⁵ defects/cm².

Electrode-defining layer 26 may be deposited using any suitabletechnique. The technique used, in part, depends on the composition ofthe electrode-defining layer. Suitable techniques include, but are notlimited to CVD, PECVD, LP-CVD, ECR-CVD, ICP-CVD, evaporation andsputtering. When the electrode-defining layer is formed of a siliconnitride material, it may be preferable to use PECVD to deposit thelayer.

Source, drain, gate and sensing electrodes may be deposited on thesemiconductor material region using known techniques such as anevaporation technique. In cases when the electrodes include two metals,then the metals are typically deposited in successive steps. Thedeposited metal layer may be patterned using conventional methods toform the electrodes.

When the sensing layer is not an electrode, the sensing layer may bedeposited using known techniques which may depend on the composition ofthe sensing layer.

Suitable techniques for forming electrode-defining layer 26 andelectrodes have been described in commonly owned, co-pending U.S. patentapplications Ser. No. 10/740,376, filed on Dec. 17, 2003, and entitled“Gallium Nitride Material Devices Including an Electrode-Defining Layerand Methods of Forming the Same”, which is incorporated herein byreference above.

It should be understood that the invention encompasses other methodsthan those specifically described herein. Also, variations to the methoddescribed above would be known to those of ordinary skill in the art andare within the scope of the invention.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A FET-based chemical sensor designed to detect a chemical speciescomprising: a semiconductor material region; a source electrode formedon the semiconductor material region; a drain electrode formed on thesemiconductor material region; a gate electrode formed on thesemiconductor material region; and a sensing region, separated from thegate electrode, and capable of interacting with the chemical species tochange a measurable property of the chemical sensor.
 2. The sensor ofclaim 1, wherein the sensing region comprises a sensing layer.
 3. Thesensor of claim 2, wherein the sensing layer has a thickness of lessthan 500 nm.
 4. The sensor of claim 2, wherein the sensing regionfurther comprises a surface region of the semiconductor material regionpositioned under the sensing layer.
 5. The sensor of claim 1, whereinthe sensing region comprises an exposed surface region of thesemiconductor material region.
 6. The sensor of claim 1, wherein thesensing region is formed between the source electrode and the drainelectrode.
 7. The sensor of claim 1, wherein a ratio of sensingelectrode surface area to total channel surface area is greater than0.35.
 8. The sensor of claim 1, wherein the sensing region comprises asensing electrode.
 9. The sensor of claim 8, wherein a separate voltagemay be applied to each of the source, drain, gate and sensingelectrodes.
 10. The sensor of claim 1, wherein respective electricalcontacts to each of the source, drain, gate and sensing electrodes areformed on a backside of the sensor.
 11. The sensor of claim 1, whereinthe sensor is substantially free of electrical contacts on a frontsideof the sensor in areas separate from the sensing region.
 12. The sensorof claim 1, wherein the semiconductor region comprises a gallium nitridematerial layer.
 13. The sensor of claim 12, wherein the sensing regionis formed, at least in part, in the gallium nitride material layer. 14.The sensor of claim 12, wherein the gallium nitride material layer has acrack level of less than about 0.005 micron/micron².
 15. The sensor ofclaim 1, further comprising a substrate.
 16. The sensor of claim 15,wherein the substrate is a silicon substrate. 17-28. (canceled)
 29. Asemiconductor device-based chemical sensor comprising: a semiconductormaterial region; a first electrode formed on the semiconductor materialregion; a second electrode formed on the semiconductor material region;a first electrical contact extending from a backside of the sensor tothe first electrode; a second electrical contact extending from abackside of the sensor to the second electrode; and a sensing region,separated from the first electrode and the second electrode, and capableof interacting with the chemical species to change a measurable propertyof the chemical sensor.
 30. The chemical sensor of claim 29, furthercomprising a third electrode, wherein the first electrode is a sourceelectrode, the second electrode is a drain electrode and the thirdelectrode is a gate electrode.
 31. A method of detecting chemicalspecies comprising: exposing a FET-based chemical sensor to a mediumcomprising chemical species; and measuring changes in drain current ofthe sensor resulting from adsorption of the chemical species on asensing region separated from the gate electrode to detect chemicalspecies. 32-39. (canceled)